Ethylene polymerization reactions with Ziegler–Natta catalysts. III. Chain-end structures and polymerization mechanism

✍ Scribed by Y. V. Kissin; R. I. Mink; T. E. Nowlin; A. J. Brandolini

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 306 KB
• Volume: 37
• Category: Article
• ISSN: 0887-624X
• DOI: 10.1002/(sici)1099-0518(19991201)37:23<4281::aid-pola4>3.0.co;2-6

✦ Synopsis

Ethylene polymerization reactions with many Ziegler-Natta catalysts exhibit a number of features that differentiate them from polymerization reactions of ␣ olefins: (1) a relatively low ethylene reactivity, (2) markedly higher polymerization rates in the presence of ␣ olefins, (3) a high reaction order with respect to ethylene concentration, and (4) a strong reversible rate depression in the presence of hydrogen. A detailed kinetic analysis of ethylene polymerization reactions 1 provided the basis for a new kinetic scheme that postulates the equilibrium formation of TiOC 2 H 5 species with the H atom in the methyl group ␤-agostically coordinated to the Ti atom in an active center. This mechanism predicts several new features of ethylene polymerization reactions, one being that chain initiation via insertion of any ␣-olefin molecule into the TiOH bond should proceed with an increased probability compared to that via ethylene insertion into the same bond. As a result, a significant fraction of ethylene/␣-olefin copolymer chains should contain ␣-olefin units as the starting units. This article provides experimental data supporting this prediction on the basis of both a detailed structural analysis of co-oligomers formed in ethylene/1-pentene and ethylene/4-methyl-1-pentene copolymerization reactions and a spectroscopic analysis of chain ends in the copolymers.